Usually, injection moulding machines have a drive, which can set a drive shaft in rotational motion and also in translational motion via a linear feed. At this shaft, a screw is attached by means of a coupling which is connected to the shaft in a torsion proof as well as a compression and tension proof manner. By means of a nozzle, the screw provides the cavity with plastics which has been supplied to the screw, controlled by the movements of the shaft connected to the screw, in order to produce plastic parts. The control of said provision must be highly precise, so that the parts which are manufactured by this process satisfy quality requirements. The control considerably depends on the pressure in the pre chamber of the screw and/or in the nozzle, respectively.
With conventional electrical injection moulding machines said nozzle pressure is indirectly measured with a fixed force sensor at or in the transmission. For each application, in particular for each machine size, a different force sensor has to be used. This arrangement has the disadvantage that the force measurement is done remote from the place where the nozzle pressure actually appears. The measurement is falsified by friction forces occurring between the measuring arrangement and the nozzle pressure.
Another well-known method for measuring the nozzle pressure comprises a nozzle pressure sensor, which is directly attached at the nozzle. This direct measurement has an optimal arrangement, however, the measurement is problematic due to the direct contact of the sensor with the melt because thereby the sensor is exposed to the high temperatures of the melt and the sensor surface further experiences an intense mechanical abrasion by the constantly passing plastic melt.
In WO 2005/002829, an arrangement is provided in which a force sensor is disposed immediately behind a feed screw in a co-rotating manner. The advantage of this arrangement is a direct measurement close to the actually arising pressure. However, this arrangement has the disadvantage that the sensor in this assembly type in addition to the axial force also experiences a torsion which is transferred to the feed screw by means of the drive shaft, which leads to a falsified measurement.